1. Field of the Invention
The present invention relates to power semiconductor modules having a wire bonding structure.
2. Description of the Related Art
Power semiconductor switching devices for use in power conversion apparatus such as an inverter include a widely known power semiconductor module which has more than one power semiconductor circuit element sealed within an electrically insulated housing. Typically a power semiconductor module is used for applications with high voltage (600 to 6,500 V) and large current (50 to 3,600 A). Such module is generally made up of a plurality of insulated gate bipolar transistors (IGBTs) and diodes, which are sealed together within the same package of such power semiconductor module. Electrode surfaces, main electrodes and control electrodes of IGBTs and diodes are connected by ultrasonic bonding techniques using diameter-increased or thick line (250 to 500 μm) aluminum bonding wires.
A major problem of the aluminum bonding wires in the power semiconductor module is that repeated flow of a large current results in a temperature rise-up and fall-down at a silicon chip and aluminum wires, which leads to creation of repeated thermal stress at the interface between the silicon chip and aluminum wire due to a difference in thermal expansion coefficient between silicon and aluminum. This stress causes unwanted progress of a crack at the interface. Thus there is a problem that the wire is physically broken and cut away in the worst case. Regarding this problem, it is shown that some improvements may be obtained by specifically designing the material and shape of such aluminum bonding wires (refer to JP-A-6-302639, for example).
In these power semiconductor modules, the rated current is determinable by Equation (1) which follows:Tj−Tc=Rthjc×Ic×Vce(Ic)  (1)where, Tj is the junction temperature, Tc is the temperature of a package housing or casing, Rthjc is the thermal resistance between the junction and the casing, Vce is the collector-emitter voltage of an IGBT, and Ic is the collector current of IGBT.
Considering about a silicon power semiconductor module which is currently the highest in current density, its loss density (loss per square centimeter) measures approximately 225 watts per square centimeter (W/cm2), whereas the current density (current flow per unit area) of IGBT is about 125 amperes per square centimeter (A/cm2).
In recent years, the loss density and the current density have been gradually improved with improvements in the junction-housing thermal resistance Rthjc and in the voltage Vce versus current Ic characteristics of IGBTs. A number of those thick-line aluminum bonding wires connectable per unit area is currently about sixteen (16) in maximum. In view of this, the flow of a conduction current per wire is 7.8 A.
Assuming that thick-line aluminum bonding wires are designed to measure 350 micrometers (μm) in diameter and 10 mm in length, a maximal level of a conduction current that is flowable per wire—known as a maximum conduction current—is 18.7 A. Accordingly, in the case of silicon power semiconductor modules, the relationship of the current density versus the maximum conduction current per bonding wire does not cause any serious problems.
However, in wide band gap semiconductor chips made of silicon carbides or else, the current density is much improved so that problems take place in the relationship between the current density and the wire's maximum conduction current. More specifically, in the case of silicon semiconductor modules, the maximum conduction current per bonding wire does not become problems at the time of proper operations. This merely pauses problems only in the event of occurrence of abnormal conditions, such as load shorting or the like. However in wide band gap semiconductor modules, a large current that is almost equal in magnitude tends to flow during normal operations. Thus, it is inevitably required to design the module structure per se by taking account of the maximum conduction current thereof. In order to increase the maximum conduction current per bonding wire, a currently available technique is to enlarge the diameter of bonding wire. Another technique is to lessen the length of such wire. Unfortunately, the former approach has a limit in increasing the wire diameter—its upper limit is approximately at 500 μm. This is due to the presence of the above-stated problems (i.e., the cracking risk due to a difference in thermal expansion coefficient, physical breakdown, and the like).